Permanent magnet rotors for dynamoelectric machines, such as electronically commutated motors, typically are constructed of a generally cylindrical iron core, which may be of a solid, laminated or sintered metal construction, and around which are positioned a plurality of magnetic elements. The magnetic elements typically are arcuate shaped with an inner contour conforming to the outer surface of the core such that the magnets may contact the outer circumferential surface of the core. The magnetic elements generally are made from barium or strontium ferrite, samarium cobalt, neodymium iron boron (rare earth) or other known magnetic materials. Permanent magnet rotors of this general type find particular application in motors having control circuitry which controls energization of the windings in one or more predetermined sequences to provide rotational magnetic fields and thereby rotation of the rotor.
The relative high mass of the materials used to form the magnetic elements and the relatively high rotor speeds create significant forces during rotor operation, including centrifugal forces and substantial momentum forces upon sudden stops or reversals of direction of rotor rotation. Retention of the magnetic elements in predetermined radial positions about the core is therefore necessary for reliable motor operation.
In addition, motors operating in fluid systems are contained in a hermetically sealed unit with the motor exposed to the fluid, such as the coolant in a refrigeration system. Recently, for example, because of increasing demand for higher efficiencies, it has become desirable to use permanent magnet variable speed motors to drive compressors in refrigeration systems. However, because many adhesives contain organic materials which are soluble in, or are leached out by, the refrigerant fluids used in these systems, resulting in matter being introduced into these fluids and condensing or otherwise accumulating in the capillary tubes and thereby causing serious or complete system failure, a motor containing minimum amounts of organic materials is desirable.
Other potential contaminants of the systems in which motors are intended for use are dust attracted by the magnetic elements and particles of the magnetic elements themselves. The magnets attract such particulate matter or, because they generally are somewhat brittle, particles of the magnetic material tend to chip or flake off during handling or use. Because dust and/or magnetic particles also can contaminate many systems in which motors are used, e.g., fluid systems and high technology applications, such as computer disk drives, encasement of the rotor assembly in a manner which eliminates the possibility of these additional potential contaminants also is desirable.
A number of methods and techniques for retaining magnets in fixed relation on the rotor core previously have been considered. One such technique has been to wrap the magnetic elements with a plastic or fiberglass material, or a relatively fine wire, and then covering the wrapping with an adhesive or epoxy resin overcoat. This technique is disadvantageous for its obvious costly labor intensive requirements and also because all of these materials become potential contaminants in any fluid system in which the rotor assembly may be used.
Another technique for retaining magnets about a rotor core involves the use of an open-ended cylindrically-shaped "can" or shell surrounding the magnets. These prior retaining shell structures typically involve deforming or press-fitting the retainer shell over the core/magnet subassembly and/or crimping or rolling the ends of the retainer shell onto the end plates, such as shown in U.S. Pat. Nos. 5,040,286 and 5,563,463, thereby requiring costly additional manufacturing apparatus and operations. Other prior structures, such as shown in U.S. Pat. No. 5,237,737, also disclose the formation of rigid ribs on the outer surface of the core which, in combination with an adhesive and/or deformation of the retainer shell, serve to prevent slippage of the magnets relative to the core. Consequently, these structures also suffer the disadvantages of manufacturing cost previously mentioned and, in addition, are incapable of being completely devoid of adhesive materials which are potential contaminants of many systems in which the rotor may be used.
In sum, while many of the aforementioned approaches to retaining magnets on rotor cores have been found satisfactory for certain intended applications, many exhibit drawbacks such as difficulty in manufacture and/or assembly, resulting in excessive manufacturing costs; failure to adequately retain the magnets in fixed radial position on the rotors during high speed operation and when subjected to repeated starting, stopping and reversal of the motor in which the rotor is used; and an undesirability for use in certain fluid systems and high technology applications, where the materials of construction in the rotor assembly are potential contaminants.